The present invention relates generally to seismic data acquisition and analysis gained through conducting a seismic survey, and more particularly to methodologies for improving the accuracy of the results based upon the seismic data analysis.
The generation and recording of seismic data involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing.
Underwater seismic exploration is widely used to locate and/or analyze subterranean geological formations for the presence of hydrocarbon reservoirs. One type of survey uses a boat towing a plurality of air guns and an array of ocean bottom nodes (OBN) placed on the ocean floor. In deep water, the nodes are placed on the ocean floor by means of a remote operated vehicle (ROV) and subsea loader; typically the ROV and subsea loader are deployed from a deployment/retrieval boat separate from the tow or gun boat. Most shallow water surveys are done deploying the nodes via rope. In addition, some surveys use ocean bottle cable (OBC) with seismometers instead of nodes. This is a similar style to OBN where the seismometers are all on the seafloor, but they are connected to the “mothership” via a long cable. The two types of acquisition styles OBN and OBC are similar in many aspects and sometimes lumped together in a more general category of ocean bottom seismic (OBS).
To acquire the data, compressed air shots are released from the air guns at known periodic intervals and the location and timing of each shot is recorded. Likewise, the timing and intensity of each compressed air shot is recorded by the nodes. In deep water, the data is typically collected for at least 30 days to cover one full tidal cycle, but may be collected over a longer period depending on, for example, the size of the area being surveyed. In shallow water, most often the nodes are on the seafloor from 5-14 days; with 21 days being considered a long deployment for shallow nodes (which typically have a maximum battery life of 45 days.)
Four-dimensional seismic data collection generally comprises three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4-D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole. Time lapse seismic data involves seismic data collection from the surface or a borehole acquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondary recovery. The data are examined for changes in attributes related to expressions of fluid content. Time-lapse seismic data can repeat 2D, 3D (which is known as 4-D seismic data), crosswell and VSP (vertical seismic profile) data.
While the direct arrival of energy at a node can be typically correctly picked when the angle of departure of the energy is less than 70 degrees, the direct arrival energy is unreliable to pick and use for positioning due to limited trace counts at the very short X, Y offsets. For example, in 100 m of water 70 degrees gives 274 m X, Y offset, at 25 m of water 70 degrees give 68 m of X, Y offset. Once you deployed to a depth of about 300 m and deeper, using the direct arrivals for positioning analysis is relatively straight forward. However, in shallow water (for example less than 300 m,) there are fewer shots to use for positioning analysis if using only direct arrivals. Furthermore, the refracted arrivals will often arrive before the direct arrival. It has been observed in most sea floor environments (but not all) that this happens at an offset around 3 times the depth of the node. Thus, while the quality of data is not affected, the reliability of using the direct arrival for positioning analysis when the angle of departure of the energy shot leaving the surface exceeds 70 degrees is adversely impacted.
In addition, the process used for deploying nodes, particularly in shallow water, gives very little control over the measurement of the water depth at each node location. Because there is local variability in local refractor velocities a method for positioning nodes must be stable in the presence of both depth of node errors and transit velocity error. Further complicating the data analysis, it is necessary to be able to detect when a node has been moved by external forces and determine correct positions of each location that a node occupied during a single deployment. In practice in shallow water surveys, the range of observed node movement has ranged from 5 m to 2000 m.
It is a primary object and advantage of the present invention to provide a method, system, and product that will improve data quality regardless of the angle at which the energy is shot from the surface.
Other objects and advantages of the present invention will in part be apparent to those of skill in the art, and in part appear hereinafter.